Impedance measuring device



EXAMINER De@ 14, 1954 a s. sHEPARD IuPsnANcE MEASURING' DEVICE 2snee-'cs sneer 1 Filed 0t. 26, 1950 ,Inverti/'o7' Y 6726 ard {Q10-vwlffys:

Dec. 14, 1954 E. s. sHEPARD 2,697,203

IMFEDANCE MBASURING DEVICE Filed oct. 26. 195o vz sheets-snee: 2

fu/VV 2^? rfsr z /MpfaA/vcf VACUUM $3/ raaf v asc/LATOR vaZrnA-TER Zjf'fnl/672507 if m'depar'd, l@ l UnitedStates Patent Oli 2,697,203Patented Dec. 14, 1954` ice IMPEDANCE MEASURING DEVICE Edward S.Shepard, Phoenix, Ariz., assignor to Trustees of Boston College, Newton,Mass.

Application October 26, 1950, Serial No. 192,274

3 Claims. (Cl. 324--S7) This invention relates to electrical measuringinstruments and more particularly to instruments of the type formeasuring electrical impedance.

motion, temperature and other external conditions, which measures of theimpedance of vacuum tube circuits under operating conditions, whichmeasures the leakage reactance and mutual inductance of a transformer,which is a precision instrument, which can be used at frequencies of 100kilocycles and greater, which measures impedances of 20,000 ohms at afrequency of one kilocycle with an error of less than 2%, which does notrequire a skilled operator, which can be used for production control,which requires only direct current elements in its circuit; which doesnot require a battery or other auxiliary source of direct potential,which is simple and economical to manufacture, which is rugged and doesnot require calibration, and which advances the electrical artgenerally.

in a broad aspect the invention contemplates a device for measuring theelectrical impedance, comprising one or more standard resistors whichare selectively connected in series with a correlated current limitingresistor and the impedance being tested between a pair of inputterminals which are adapted to be connected to a source of alternatingpower such as an oscillator or a sweep circuit generator. By connectinga selected one of a plurality of standard resistors, which hasapproximately the same ohznic value as the impedance being tested, therange of the device is greatly extended without sacrifice of accuracy.The accuracy of the device is furtherincreased by changing the currentlimiting resistor when the standard resistor s changed so that the ratioof the ohmic value of the current limiting resistor to the standardresistor remains a constant, for example 100:1 regardless of thestandard resistor selected. Such changing of the resistors is preferablycarried out simultaneously for example by plugging or the use of amultiposition gang range selector switch having two switch units orelements each of which comprises a plurality of stationary contacts anda movable contact, the movable contacts being mechanically linked sothat they move simultaneously to the corresponding switch positions. Oneend of each 0f the standard resistors is connected to a respectivestationary contact of one of the switch units. The movable contact ofthe same switch unit is joined to one end of each of the currentlimiting resistors. The opposite end of each limiting resistor isconnected to a corresponding stationary contact of the second switchunit whose movable contact is connected through the test impedance toone input terminal connecting to the power source. The opposite end ofeac'n of the standard resistors is connected to the other input terminalso that`the power source is connected in series with the test impedance,a standard resistor and its correlated current limiting resistordetermined bv the position of the gang switch.

Switch means are provided for selectively connecting a sensitivevoltmeter. for example a` cathode ray oscillograph or vacuum tubevoltmeter to measure the individual and total voltage drops across thestandard resistor and vtively to the terminals of the voltmeter.

switching means is a four pole three position gang switch with fourseparate switch units or elements. Each of the switch elements has threestationary contacts. and an associated movable contact which moves intocorresponding positions conjointly with the three movable contacts ofthe other associated switch elements. The movable contact of the firstswitch element is directly linked to the movable element of the secondswitch unit of the range selector switch so that it can be selectivelyconnected to the desired standard resistor. The movable contact of thesecond switch element is connected to the ends of the standardresistors. The first stationary contact of the second switch element andthe second and third stationary contacts of the rst switch element areconnected to one of the input terminals of the power source. The otherinput terminal is connected through the impedance being tested to boththe second and third stationary contacts of the second switch elementand the first stationary contact of the first switch element so that ineach position of the voltameter switch the circuits completed betweenthe power source input terminals have substantially the same totalimpedance thus maintaining a substantially constant current flow throughthe test impedance. The third and fourth switch elements of thevoltmeter switch are used to connect the voltmeter so that the separateor individual voltage drops across the selected standard resistor andtest impedance and the total voltage drop across the standard resistorand the impedance when connected in series are measured thereby. To thisend the movable contacts of the third and fourth switch elements areconnected Vrespec- The first and third stationary contacts of the thirdswitch element are joined to the common connection to the currentlimiting resistors and the movable contact of the first switch unit ofthe range selector switch. The first stationary contact of the fourthswitch element and the secondpstationary contact of the third switchelement are connected to the movable contact of the second switchelement which is also linked to the common ends of the standardresistors whereby the three positions of the voltmeter switch connectthe voltmeter so that it measures respectively the voltage drop acrossthe selected standard resistor, the voltage drop across the testimpedance, and the total voltage drop across the resistor and impedance.

These and other objects and aspects will be apparent from the followingdescription in an illustrative specic engbtlndiment of the inventionreferring to a drawing in w ic Fig. l is an isometric view of theimpedance measuring device;

Fig. 2 is a wiring diagram of one embodiment of thev device; and

Fig. 3 is a wiring diagram of a second embodiment.

In the first embodiment of the invention chosenv for the purposes ofillustration, the test impedance Zt (Fig. 2) is connected by means oftwo terminals such as binding posts Z1 and Z2 which are convenientlylocated upon the top of one end of a cover 12 for a casing 10 such as isshown in Fig. l. At the opposite end of the cover 12 are pro'- vided4four additional binding posts, one pair of which are designated t1 andt2 andare used as terminals for connecting a source of alternating powersuch as an oscillator the test impedance. In a more specific aspectsuchor sweep circuit generator. The second pair of binding posts Il? andt4 accommodate the leads from a voltage measuring device such as avacuum tube voltmeter (Fig.

2). Also extending from the top of the cover 12 are two handles or knobs14 and 16 which with their associated' shafts (not shown) are actuatorsfor transferring respectively the contacts of a range selector switchand a voltmeter switch as will be described in detail hereinafter.

The range selector switch comprises two electrically isolated switchunits SR and SI. Each of the switch units comprises five stationarycontacts which are selectively contacted by a movable contact tocomplete an 'electrical circuit-therebetween. Each .of the movablecontacts is' operated conjointly with the'other movable contact by meansof the actuator 14 so that corresponding stationary contacts'of bothswitch units are contacted simultaneously. Each of the stationarycontacts of the Witch unit SR is connected to one end of a respectiveone of live standard resistors Rl-RS which are accurately calibrated andused as the basic measuring standards. The movable contact of the switchunit SR is connected by means of the conductors 20 and 22 to one end ofeach of tive current limiting resistors designated rl-rS in Fig. 2. Theopposite end of each of the current limiting resistors r1--r5 isconnected to a respective stationary contact of the switch unit SI. Fromthe above it will be apparent that the operation of the ganged switchunits SR and SI of the range selector switch by means of the actuator 14connects a selected one of the standard resistors R in lseries with acorresponding current limiting resistor r.

The opposite ends of the standard resistors R1-R5 from those endsconnected to the stationary contacts of the switch unit SR as describedabove are connected together by means of a common lead 24 which also islinked with the movable contact of a switch element S2v of the voltmeterswitch. The movable contact of the switch unit SI is directly linked tothe movable contact of the voltmeter switch element S1 by means of ashielded conductor 26. The stationary contacts designated Z and T of theswitch element S1 and the stationary contact R of the switch unit S2 areconnected by means of a shielded conductor 28 with the input terminal t1which is adapted also to connect with an oscillator or other alternatingpower source. The stationary contact R of the switch element S1 and thestationary contacts Z and T of the switch element S2 are linked by aconductor 29 to the terminal z1 of thetest impedance Zt. The otherterminal z2 of the test impedance Zt connects to the other inputterminal t2 by means of the conductors 30 and 32. ln addition totheshjelding of the conductors 26 and 28, shields such as the groundedmetal plates 31 are interposed between the switch elements S2 and S3 andthe switch elements S1 and S4 thereby to further reduce straycapacitance coupling.

From the above it will be apparent that when`the actuator 16 of thevoltmeter switchis in the position shown wherein the movable contacts ofthe switch elements S1 and S2 `contact the respective stationarycontacts Z,.a circuit is completed between the input terminals 11 'and`t2 which includes the conductor 28, the contacts of the switch unit ST;the current limiting resistor rl--rS determined by the position of therange selector switch, the conductors 22 and 20, the contacts of theswitch unit SR, the selected standard resistor Rl-RS which is alsodetermined by the position of the range selector switch, the conductor24. the contacts of switch element S2, the conductor 29, the testimpedance Zt and the conductors 30 and 32 to the oscillator terminal t2.When the voltmeter switchl actuator 16` is in such a position that themovable contacts of the switch elements S1 and4 S2 are in positionto-make with the respective stationary contacts designated R, 'a circuitis completed from -the inout terminal t1 throuwh the conductor 28, thecontacts of the switch element S2: the conductor 24,- the selectedstandard* resistor Rl-RS. the range selector switch unit. SR. theconductors 20 and 22, the corresponding current limiting resistor rl-rS,the range selector switch unit ST, theconductor 26, the switch elementS1." the conductor 29, the testimpedance Zt an the conductors 3 0 and 32to the inout terminal t2. When the voltmeter switch actuator 16 is in aposition wherein the movable contacts of the switch elements S1 and S2contact the respective stationary contact designated Ta circuit iscompleted including the input terminal 11,-the conductor 28, thecontacts of the switch element S1, the conductor 26. the range selectorswitch unit ST, the selected current limiting resistor r1--r5, theconductors 22 and 20. the range selector switch unit SR, the standardresistor Rl-RS, the conductor 24. the voltmeter switch element S2. theconductors 30 and 32 to the input terminal t2. From the above it will beapparent that it! @ach of the series circuits completed between theinput terminals t1 and t2 in each of the three respective positions ofthe voltmeter switch the total series impedance is substantially thesame, and that therefore with a constant voltage power source thecurrent through the test impedance Zt is independent of the position ofthe voltmeter switch.

The terminals t3 and t4 which are adapted to connect with the leads tothe voltmeter are also coupled by the conductors 34 and 36 with themovable contacts of the voltmeter switch elements S4 and S3respectively. The stationary contacts designated R and T of thelvoltmeter switch element S3 are coupled to the conductors 20 and 22which complete the circuit between the movable con tact of the rangeselector switch unit SR and the common ends of the current limitingresistors r1--r5. The stationary contact Z of the switch element S3 andthe stationary contact R of the switch element S4 are connected by meansof a conductor 38 with the conductor 24 which extends between the commonends of the standard resistors Rl-RS and the movable contact of theswitch element S2 as has been described heretofore. The stationarycontacts Z and T of the switch element S4 are connected by means of theconductor 32 with the input t2.

From the above it will be apparent that with the voltmeter switch.actuator 16 in the position wherein the movable contacts make with thestationary contacts R,- the voltmeter terminal t3 is connected with thecommon end of the standard resistors R2-R5 by means of a circuitincluding the conductor 34, the switch element S4, the conductor 38,andthe conductor 24. The voltmeter terminal t4 is connected withtheopposite end of the selected resistor Rl--RS by means of a circuitincluding the conductor 36, the switch element S3, the conductor 20 andthe selector switch unit SR. In other words with the voltmeter switch inthe position designated `R, the voltmeter reads the voltage drop acrossthe selected standard resistor Rlv-RS. With the voltmeter switch in theposition designated Z the terminal t3 is connected with the terminal Z2of the test impedance Zt by means of a circuit including the conductor34, the switch element S4, and the conductors 32 and 30. The voltmeterterminal t4 is connected with the opposite terminal Z1 of the testimpedance Zt by means of a circuit including the conductor 36, theswitch element S3. the conductors 38 and 24, the switch element S2 andthe conductor 29 so that the voltmeter reads the voltage drop across thetest impedance. With the voltmeter switchin position so that the movablecontacts make with the stationary contacts designated T, the voltmeterterminal t3 is again connected with the terminal Z2 of thetest-impedance Zt by means of a circuit similar to that described aboveand including the conductor 34, the switch element S4, and theconductors 32 and 30. The voltmeter terminal t4 is connected with theend of the selected standard resistor Rl-RS by means of a circuitincluding the conductor 36, the switch element S3, the'conductor 20 andthe selector switch unit SR so that the voltmeter indicates the totalseries voltage drop across both the selected standard resistor Rl-RS andthe test impedance Zt.

As described above, the test impedance element Z whose characteristics-are to be measured is connected to the terminals Z1 and Z3. A voltmeter(or cathode ray oscilloscope) having a high inout impedance A(500k orgreater) is coupled to the output terminals t3 and t4` the inputterminals t1 and t2 being connected to an alternating power source suchas an oscillator. A standard resistor from the group of resistors Rl-RShaving an ohmic value approximating that of the test impedance Zt isselected empirically by means of the range selector,

switch. the resistor selected being that which givesthe least diterencein the voltmeter readings` as the voltmeter switch is transferred fromthe Z position to the R position so that the scale error of thevoltmeter is minimized.

As the current through the selected standard resistor Rl--RS and vthetest impedance Zt remain constant irrespective of the position of thevoltmeter switch. as discussed in detail heretofore, the followingrelation` ship holds:

I=Er/R=Ez/Zt (1) and where:

Zt=the impedance value of test impedance R=the resistive value ofstandard resistor Ez=the potential drop across the test impedance Er=thepotential drop across the standard resistor It will be apparent that Et'and Ez are read directly upon the voltmeter when the voltmeter switch isin the positions designated R and Z respectively. lf the current I isregulated for example by changing the output voltage of the power sourceor by means of an auxiliary regulating potentiometer (not shown) so thatthe ratio of R/'Er is equal to unity, the ohmic value of the impetlanceZt can be read directly upon the voltmeter scale. Otherwise the valuesof Er and Ez obtained byreading the voltmeter and the value of Robtained from the scale upon the top of the cover associated with theknob of range selector switch actuator 14 must be substituted in theabove Equation 2 so that the equation can be solved for Z. The value oftotal series voltage drop Et across the standard resistor and the testiinpcdA ance is read upon the voltmeter when the voltmeter switch is inthe position T so that when desired the resistive and reactivecomponents of the impedance Zt can be determined either graphically oralgebraically from the vector expression:

The above mentioned measurements can also b c made by means of a secondembodiment whose circuii components are connected according to thewiring diagram shown in Fig. 3. In this second embodiment the standardresistors Rl-RS and the current limiting resistors' rl-rS areselectively connected by means of a range selector switch having twoswitch units SR and Sl so that the ratio of the ohmic values ofcorresponding standard and current limiting resistors remains a constantas was described in detail heretofore with respect to the firstembodiment of the invention. In the second embodiment the voltmeterswitch has three switch elements sS, s6 and 57 operated conjointly, sothat the movable contact of each makes selectively with three stationarycontacts designated R, T and Z respectively.

The joined ends of the current limiting resistors rl-rS are connected toan alternating power source such as an oscillator by means of an inputterminal tl. The other input terminal t2 is connected with the contactsT and Z of the rst voltmeter switch element .v5 and the contact R of thesecond switch element s6. The movable contactof the tirst selectorswitch unit Si' is connected with the contacts Z and T of the secondswitch element s6 and the contact R of the first switch element s5. Themovable contact of the second switch unit SR connects with thestationary contacts R and Z of the third voltmeter meter switch element.i7 and with the terminal z2` of the test impedance Zt. The otherterminal zl of the test impedance Zt is linked with the movable contactof the first switch element 55, The lead which joins the ends of thestandard resistors Rl--RS branches to connect with the stationarycontact T of the third switch element s7 and the movable contact of thesecond switch element s6. The terminals' t3 and t4 for connecting with avacuum tube voltmeter nre coupled respectively to the movable contact ofthe third switch element s7 and the input terminal t2.

lfVhen the voltmeter switch in the above described Aembodiment is in theR" position, a circuit is completed between the .oscillator terminalswhich includes the terminal t1, the current limiting resistor r1-r5 determined by the position of the range selector switch unit Sl, thevoltmeter switch element sS, the test impedance Zt, the correlatedstandard resistor Rl-RS determined by the position of the range selectorswitch unit SR, and the switch element s6 to the terminal t2. The switchelement .i7 conjointly connects the terminal t3 of the voltmeter withthe common connection between the test impedance terminal z2 and themovable contact of the switch unit ySR so that the meter measures thevoltage drop across the selected standard resistor. By shifting thevoltmeter switch to the position Z the same circuit elements arereconnected to complete a circuit including t1, the selected currentlimiting resistor r1-r5, Sl, s6. the corresponding standard resistorRl-RS, SR, Zt and sS 'to the terminal t2. It will be noted that theseare equivalent identical circuit components to those in position R butreconnected in a different series sequence so that the current tlowremains substantially constant in either position of the voltmeterswitch. The voltmeter switch element connects the terminal t3 of thevoltmeter to the common point between the movable contact of the switchunit SR and the test impedance terminal z2 so that the meter isconnected in parallel with the test impedance Zt. In the T position ofthe voltmeter switch, the sequence of the components in the seriescircuit between the oscillator terminals t1 and t2 is not changed, butthe switch element S7 reconnects the voltmeter terminal t3 to the commonpoint between the ends of the standard resistors R1-R5 and the movlablecontact of the switch element s6 so that the meter reads the totalvoltage drop across the test impedance Zt and the selected standardresistor.

It should be understood that the present disclosure is for the purposeof illustration only and that this invention/includes all modificationsand equivalents which tall within the scope of the appended claims.

I claim:

l. An impedance measuring device comprising a pair of terminals adaptedto be connected to an alternating power source, a standard resistor, acurrent limiting resistor, means for directly and non-reactivelyconnecting the impedance to be measured and said terminals in se rieswith the standard resistor and the current limiting resistor, andswitching means for selectively connecting directly and non-reactively avoltmeter across the standard resistor, across the impedance to bemeasured, and across the standard resistor and the impedance to bemeasured in series, respectively, to measure the individualand totalvoltage drops.

2. An impedance measuring deviceA comprising a pair of terminals adaptedto be connected to an alternating power source, a plurality of standardresistors, a plurality of current limiting resistors each of which iscorrelated with one of the standard resistors, means for directly andnon-reactively connecting the impedance to be measured and saidterminals in series with a selected standard resistor and its correlatedlimiting resistor,'and switching means for selectively directly andnon-reactively connecting a voltmeter across the selected standardresistor, across the impedance to be measured, and across the selectedstandard resistor and the impedance to be measured in series,respectively, to measure the individual and total voltage drops.

3. An impedance measuring device comprising a pair of terminals adaptedto be connected to an alternating power source, a plurality of standardresistors, a plurality of current limiting resistors, a range selectorswitch for selectively'connecting each of the standard resistors with arespective current limiting resistor having an ohmic value Vot' at leastone hundred times as large as that of theconnected standard resistor,means for connecting directly and non-reactively the impedance to bemeasured and said terminals in series with a selected standard resistorand its correlated limiting resistor, and switching means forselectively connecting a voltmeter across the selected standardresistor, across the impedance to be measured, and across the selectedstandard resistor and the impedance to be measured in series,respectively,

.to measure the individual and total voltage drops.

4. An impedance measuring device comprising a pair of input terminalsadapted to be connected to an alternating power source, a range selectorswitch including two coniointly operable switch units each of whichunits has a plurality of stationary contacts and a movable con-l tactfor selectively making contact therewith, a plurality of standardresistors one end of each of which is connected to a respectivestationary contact of the first switch unit, a plurality of currentlimiting resistors one enll of each ofwhich is connected to arespectivestaticnaw contact of the second switch unit, the other endsci" the current limiting resistors being connected to the movablecontact of the first switch unit whereby the coi*- related currentlimiting resistors can be connected ii series with the selected standardresistors, means including the opposite ends of the standard resistorsand the movable. contact of the second switch unit for directly andnon-reactively completing a series circuit including the inputterminals, the impedance being tested, the selected standard resistorand its correlated limiting resistor, and switching means forselectively connecting directly and non-reactively a voltmeter acrossthe se-l lected standard resistor,. across the impedance to be measured,and across the selected standard resistor and the impedance to bemeasured in series, respectively, to measure the individual and totalvoltage drops.

5. An impedance measuring device comprising a pair of input terminalsadapted to be connected to an alternating power source; a standardresistor; a current limiting resistor connected in series with thestandard resistor; a four pole three position voltmeter switch includingfour coniointly operable switch elements each of which has threestationary contacts and a movable contact for selectively making contacttherewith, the movable contacts of the first and second switch elementsbeing directly and nonreactively connected respectively to one end ofthecurrent limiting resistor and one end of the standard resistor, thefirst stationary contact of the second switch element and the second andthird stationary contacts ot.7 the first switch element being directlyand non-reactively connected with the first of said input terminals, thesecond input terminal being directly and non-reactively connectedthrough the impedance being tested to the second and third stationarycontacts of the second switch element and the first stationary contactof. the first switch element so that in all three positions of theswitch, respective circuits arc completed between the input terminalswhich have substantially the same total value of electrical impedancethus maintaining the current through the test impedance substantiallyconstant, the tirst and third stationary contacts of the third switchelement being joined to the common connection between the standardresistor and the current limiting resistor, and the first stationarycontact of the fourth switch element and the second stationary contactof the third switch element being direcly and non-reactively connectedto the movable contact of the second switch element; and terminals fordirectly and non-reactively connecting the movable contacts of the thirdand fourth switch elements to a voltmeter, whereby the three positionsof the switch connect the voltmeter to measure selectively theindividual and total voltage drops across the standard resistor and thetest impedance.

6. An impedance measuring device comprising a4 pair of input terminalsadapted to be connected to an alternating power source; a range selectorswitch including two Yswitch units conjointly operable each of whichunits has a plurality of stationary contacts and a movable contact forselectively making contact therewith; a plurality of standard resistorsone end of each of which is directly and non-reactively connected to arespective stationary contact of the rst switch unit; a plurality ofcurrent limiting resistors one end of each of which is directly andnon-reactively connected to a respective stationary contact of thesecond switch unit, the other ends of the current limiting resistorsbeing directly and non-reactively connected to the movable contact ofthe first switch unit whereby the correlated current limiting resistorscan be directly and non-reactively connected in series with the selectedstandard resistors; a four pole three position voltmeter switchincluding four conjointly operable switch elements each of whichelements has three stationary contactsand a movable contact forselectively making contact therewith, the movable contact of the firstelement being directly and non-reactively connected to 'the movableContact of the second selector switch unit, the movable contact of thesecond voltmeter switch element being directly and non-reactivelyconnected to the opposite ends of the standard resistors to the endsconnecting with the stationary contacts of the first selector switchunit, the first stationary contact of the second switch element and thesecond and third stationary contacts of the iirst switch eiements beingdirectly and non-reactively connected with the first of said inputterminals, and the second input terminal being directly andnon-reactively connected through the impedance being tested to thesecond and third stationary contacts of the second switch element andthe first stationary contact of the first switch element, so that in allthree positions of the voltmeter switch respective circuits arecompleted between the input terminals which have substantially the sametotal value of electrical impedance thus maintaining the current throughthe test impedance substantially constant, the first and thirdstationary contacts of the third switch element being joined to thecommon connection between the ends of the current limiting resistors,the first stationary contact of the fourth switch element and the secondstationary contact of the third switch unit being -connected to themovable contact of the second switch element; and terminals for directlyand non-reactively connecting the movable contacts of the third andfourth switch elements to a voltmeter, whereby the three positions ofthe voltmeter switch connect the voltmeter to measure selectively theindividual and total voltage drops across the selected standard resistorand the test impedance.

7. An impedance measuring device comprising a pair of input terminalsadapted to be connected to an alternating power source; a currentlimiting resistor; one end of which is directly and non-reactivelyconnected to one of said input terminals; a three pole, three positionvoltmeter swtch including three `conjointly operable switch elementseach of which has three stationary contacts and a movable contact forselectively making contact therewith, the opposite end of the currentlimiting resistor being directly and non-reactivcly connected to thefirst stationary contact of the first switch element and the second andthird stationary contacts of the secondI switch element, the firststationary contact of the` second switch element and the second andthird stationary contacts of the first switch element being directly andnon-reactively connected to the other of said input terminals; astandard resistor, one end of which is directly and non-reactivelyconnected to the movable contact of the second switch element and thethird contact of the third switch element,

non-reactively connected to the first and second stationary contacts ofthe third switch element and one end of the impedance being tested, andthe movable contact of the first switch element being directly andnon-reactively connected to the other end of the impedance being tested;and two voltmeter terminals, one of which is directly and non-reactivelyconnected to the movable contact of the third switch element and theother of which is directly and non-reactively connected to said otherinput terminal, whereby the three positions -of the voltmeter switchconnect the voltmeter to measure selectively the individual and totalvoltage drops across the standard resistor and the test impedance.

8. An impedance measuring device comprising a pair of input terminalsadapted to be connected to an alternating power source; a two polemultiposition range selector switch including two switch units each ofwhich has a plurality of stationary contacts and a movable contact forselectively making contact therewith; a plurality of current limitingresistors, one end of each of which lit) is directly and non-reactivelyconnected with a respective stationary contact of the first switch unit,`the oppositel ends of the4 resistors being directly and non-reactivelyconnected with one of the input terminals; a plurality of standardresistors, one end of each of which is directly and non-reactivelyconnected with'a respective stationary contact of the second switchunit; a three pole, three position voltmeter switch including threeconjointly operable switch elements each of which has three stationarycontacts and a movable contact for selectively making contact therewith,the movable contact of the first switch unit being directly andnon-reactively connected to the first stationary contact of the firstswitch element and the second and third stationary contacts o t thesecond switch element, the iirst stationary contact of the second switchelement and the second and third stationary contacts of the first switchelement being directly and non-reactively connected to other of saidinput terminals, the opposite ends of the standard resistorsbeingdirectly and non-reactively connected to the movable contact of thesecond switch element and the third contact of the third switch element,the movable contact of the second switch unit being directly andnon-reactively connected to the first and second stationary contacts ofthe third switch element and one end of the impedance being tested, andthemovablecontact of the first switch element being directly andnon-reactively connected to the other end of the impedance being tested;and two voltmeter terminals, one of which is directly and non-reactivelyconnected to the movable contact of the third switch element, the otherof which is directly and non-reactiyely connectedl to said -other inputterminal, whereby the three positions of the voltmeter switch connect'the voltmeter to measure selectively the individual and total voltagedrps across the standard resistor and the correlated test impedancedetermined by the position of the range selector switch.

(References on following page) l0 References Cited in the le of thispatent burle P llzlmle al A lasue ac ar et p1'. 1952 UNITED STATESPATENTS 2,626,981 shiepe Jan. 27, 1953 Number Name Date 899,193 shinesept. 22, 1908 5 OTHER REFERENSES 1,684,403 Mason Sept. 18,1928 Famer:Electrical Measurements in Practice, Mc- 1,9l7,417 Zuschlag July 11,1933 Graw-Hill Book Co., rst edition, 1917, page 265.

2,547,926 Cook Apr. 10, 1951

